The present invention relates to a new and improved apparatus for the transmission end-enciphering and reception end-deciphering of information.
The apparatus of the present invention is generally of the type comprising an enciphering device arranged at the side of the transmitter which receives the plain text in the form of information signal groups, separated from one another by separation or partition signals, and is connected with a key character-information storage which delivers to the enciphering device the key character-information needed for enciphering. There is further provided a deciphering device arranged at the side of the receiver, which deciphers the received information and is connected with a key character-information storage which delivers key character-information needed for deciphering to the deciphering device. Both at the key character-information storage at the transmitter and at the key character-information storage at the receiver there are stored at least two different key character-information, wherein selectively in each case one such type of key character-information can be entered to the enciphering device and deciphering device respectively.
In the telecommunications art, information is frequently transmitted in a binary code in the form of pulse trains. A classic example for such information transmission is the CCITT-Code 2 used in teleprinter networks. With this code each character is represented by a combination of 5 binary pulses. With a start-stop technique there is introduced before and after each pulse group representing a character a start step and a stop step, so as to thus ensure for synchronisation of the transmitter equipment and receiver equipment.
The present day regulations require that there be transmitted by the transmitter equipment a stop step which has a length amounting to at least 1.4-fold that of an information step and that the receiver end-equipment can still detect and process steps of 0.8-fold length as stop steps.
While taking into account these characteristics it is possible at the present time to design transmission systems which synchronously operate in a reliable manner. Such type transmission system has been disclosed, for instance, in U.S. Pat. No. 3,702,900, to which reference may be readily had and the disclosure of which is incorporated herein by reference.
During the synchronous operation of transmission systems of this type, there are no longer required all of the start-stop steps for ensuring synchronisation of the system.